1. Field of the Invention
This invention relates to an automatic focus control system for video cameras in which the effective diameter of the diaphragm of the optical sub-system is changed in dependence upon the focused state of the optical main system for detecting a focused point for the optical main system and the direction in which the main system is defocused.
2. Description of the Prior Art
Various methods are known in the art for detecting a focused point for effecting automatic focus control of an optical system used in a video camera or an optical film camera, such as the active type detecting method which makes use ultrasonic sound or infrared light projected to and reflected back from the object, or the passive type method which makes use of natural light reflected back from the object.
When an ultrasonic wave is used for detecting focusing with respect to a distant object, it is difficult to effect precise detection because of the lower directivity of the ultrasonic wave. When an infrared light is used, power consumption of the infrared light source is considerably increased, which is not desirable for a video camera which makes frequent use of a lens with a larger zoom ratio. In the passive type detecting method, there are known a correlation method in which the object image is split and the correlation of the resulting images is scrutinized and a sharpness method in which sharpness of the object image is used for detecting focusing of the optical system. These methods are devoid of the inconveniences as not in the passive type method because natural light is used. However, in the correlation method, sensors of delicate and complex construction are required thus causing elevated costs. In addition, there is the risk of malfunction when the object is complex or the object image consists of a repetition of patterns. In the sharpness method, it is difficult to precisely determine the direction in which the optical system is deviated from the focused point. It is also known to combine the correlation and sharpness methods; however, there is an inconvenience that the method requires a complicated optical system and a complicated processing circuit for output signals.
In still cameras taking shots of still images, correct shooting can be achieved by operating the shutter in the correctly focused state of the optical system, even when the automatic focus control system shows only poor response properties. However, in video cameras taking a sequence of consecutive shots, imaging signals are outputted even when the optical system is in the defocused state and the images of inferior quality are reproduced on, for example, the screen of a monitor television receiver. Thus in effect, the quality of the output image is governed directly by the properties of the automatic focus control system.